A spirometer is a device that monitors respiration. A spirometer can be used for the diagnosis and monitoring of pulmonary diseases, particularly asthma and COPD (smoker's cough). Spirometers are also used to monitor the performance of athletes, as well as screen for occupational health problems such as black lung disease or silicosis. Spirometers are generally divided into two classes which have different specifications and purposes. The first class of spirometer are diagnostic spirometers which are used by physicians for the diagnosis of a person's respiratory condition. Diagnostic spirometers must be able to measure many different parameters of respiratory flow and must have a high degree of accuracy. The second class of spirometers are monitoring spirometers which are used to monitor the condition of the lungs on a regular basis. Spirometers used for monitoring must be inexpensive, portable, and easy to use.
Monitoring spirometers usually measure a key parameter called peak respiratory flow or peak expiratory flow (PEF). Peak expiratory flow (hereafter referred to as “peak flow”) is defined as the maximum flow rate recorded during a forced expiration of air from the lungs. A person's respiratory condition can be monitored by measuring peak flow with a portable spirometer. Doctors recommend that patients with moderate to severe asthma should record their peak flow on a daily basis to determine the effectiveness of the treatment given to them. This opinion is also supported by the US government sponsored National Heart, Lung, and Blood Institute (NHLBI). When a patient is able to regularly monitor his/her condition, the chances of successful treatment are improved.
Most monitoring spirometers are mechanical devices with a moving vane or rotating wheel that record the air flow caused by a person's expiration into the spirometer. While these spirometers are inexpensive and, therefore widely used, they suffer from certain disadvantages. Typically, these spirometers have a low level of accuracy due to friction and other artefacts of their mechanical construction. Other limitations arise because the inertia of the mechanical vane or wheel prevents a reliable flow measurement as a function of time, and so the spirometers are only capable of measuring an approximation of the peak flow. In addition, ordinarily such spirometers have no means of recording and transmitting the results in electronic form. This means that the patient must record results manually. In some cases, electronic sensing is used to record the motion of a vane or wheel, and the results can be stored internally or sent electronically to remote computers. However, the underlying mechanical means of flow measurement prevents a highly accurate or complete determination of the flow properties.
In order to determine if a peak flow result from a monitoring spirometer is reliable, it is necessary to observe the overall flow versus time during the entire breathing manoeuvre. If a user does not use a correct breathing technique, coughs, or does not exhale forcefully enough, the peak flow meter will have an inaccurate result. Only by observing the resulting flow versus time data would a physician be able to determine if the particular manoeuvre was acceptable. Because of the limitations described above, existing mechanical monitoring spirometers are not able to make this determination.
Some purely electronic spirometers (pneumotachographs) were developed which calculate the air flow from a pressure difference measured across an obstruction in the flow channel. Most often, a differential pressure sensor is connected to two outlets on the flow channel on either side of the flow obstruction. The obstruction may be a restriction in the flow channel or a fine wire mesh or ceramic screen. These spirometers are an improvement over the mechanical spirometers but still have certain drawbacks. The restriction or screen may trap contaminants from the user's breath which could alter the flow properties of the spirometer. These contaminants may also spread disease from one user to another and the spirometer must be carefully sterilized. The ports in the flow channel that connect to the pressure sensors must be kept clear of contaminants that impede the flow and could damage the sensor. Sterilization of the ports must also be possible without damaging the pressure sensors. For these reasons many pneumotachographs provide filters or membranes to protect the sensors, but these add to the complexity of the spirometer and reduce its sensitivity to air flow. The pressure transducers, which are used to sense the pressure on either side of the restriction, are often of an expensive design making them too costly to be used widely as monitoring spirometers. For the most part, pneumotachographs are sold as diagnostic spirometers for use by medical professionals rather than by the general public.
The pneumotachographs intended for portable monitoring typically calculate, store, and display on an LCD display only the value of the peak flow. This limitation results because the processors and memory used in these electronic spirometers have a limited processing speed and size, and are unable to make accurate determination of flow versus time in real time. A hardwired ROM memory on such pneumotachographs is an additional drawback because it requires physical replacement of the memory spirometer to accommodate an improved or customized data acquisition algorithm. The data are also stored in RAM memory and, as a result, are lost when power is interrupted. Due to the low sensitivity of the spirometer, heavy analog filtering is required. Such filtering lowers the time response of the spirometer, and results in less accurate data being recorded.
To be of any value, the results obtained from a spirometry measurement require a proper effort and technique from the user during the forced expiration. A peak flow result obtained with the wrong technique is useless, and so the technique used must be monitored. Preferably, a doctor should be able to view the entire flow versus time chart to determine if the technique was correct and, hence, whether or not the flow result should be considered.
Advances in microprocessor and memory technology as well as improved solid state pressure sensors enable new monitoring spirometers with improved features and lower cost. New spirometers using EEPROM (flash) memory would permit the remote programming of the spirometers. This would allow practitioners to adapt their algorithm for the flow measurement to best suit individual conditions and even individual users. With the rise of the internet and desktop computing, a spirometer designed to be interfaced with a portable computer is also highly desirable and should improve patient care and monitoring possibilities.
The flow rate and flow volume results obtained by pneumotachographs are dependent on local temperature and atmospheric pressure. The local atmospheric pressure varies on the order of 10% due to weather fluctuations and may change even more significantly due to the elevation (e.g., there is an 18% air pressure variation between sea-level in San Francisco and Boulder, Colo. at 1,500 m). Few spirometers correct for this automatically, and a manual correction must usually be done after an independent measurement of the local pressure and temperature. If the spirometer is to be used by a patient at home this type of manual correction is not convenient or practical.
There is a clear need for a purely electronic monitoring spirometer that provides reliable results with a low cost design. The design should be as simple as possible to reduce the effects of contamination and allow sterilization. The spirometer should be sufficiently easy to use for patients themselves to perform home monitoring. It should not require extensive maintenance. The spirometer should also be capable of interfacing with a desktop computer or the internet to allow convenient data collection. Collection of the entire flow versus time waveform is also desirable so that medical professionals can check the reliability of the results. An optional additional feature of such a spirometer would be a feature that measures the local temperature and atmospheric pressure and makes an automatic correction of the results. Such a feature would provide a significant enhancement of the spirometer's accuracy